Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder. Direct fuel injection systems may improve cylinder charge cooling so that engine cylinders may operate at higher compression ratios without incurring undesirable engine knock. Port injection systems may reduce particulate emissions and improve fuel vaporization. In addition, port injection may reduce pumping losses at low loads. To leverage the advantages of both types of fuel injection, engines may also be configured with each of port and direct injection. Therein, based on engine operating conditions, such as engine speed-load ranges, fuel may be delivered via only direct injection, only port injection, or a combination of both types of injection.
The inventors herein have recognized potential issues that may occur when operating with only port injection. Specifically, when port injection is scheduled, fuel may be delivered via a port injector only within a defined window that starts shortly after an intake valve closes and ends just before, or shortly after, the intake stroke. If a tip-in occurs late in this cycle (e.g., towards a later part of the port injection window), the estimated air charge entering the cylinder will rise rapidly. An engine controller may react to this rise in estimated air charge by estimating a corresponding increase in fuel required to maintain stoichiometric engine operation. However, there may not be sufficient margin to enable the additional fuel to be delivered before the port fuel injection window ends. As a result of the port injection error, a lean combustion event may ensue, increasing the chance for engine misfires.
The inventors herein have recognized the above issues and developed a method for an engine to at least partly address some of the above issues. One example method includes: operating in a first mode with each of a port and a direct injector enabled, operating in a second mode with the port injector enabled and the direct injector disabled, wherein the direct injector is selectively re-enabled responsive to the port injection fuel error, the error then compensated via each of port injection and direct injection on a common combustion event; and operating in a third mode with the port injector enabled and the direct injector disabled, wherein the direct injector is selectively re-enabled responsive to the port injection fuel error, the error compensated via only direct injection on the common combustion event. In this way, stoichiometric engine operation is improved.
As one example, during conditions where only port injection is scheduled (e.g., low engine speed-load conditions), delivery of fuel pulses from cylinder direct injectors may be inhibited and a target fuel mass may be delivered via a cylinder port injector. In particular, the port injection may be scheduled with a start and end of injection timing within the port injection window. In response to a tip-in event occurring while the port injection is in progress, a controller may calculate an additional amount of fuel required to be delivered to maintain stoichiometric combustion. The controller may then determine if the additional fuel mass can be delivered by adjusting the port injection pulse width (e.g., by extending the end of injection timing) within the port injection window. If the fuel error cannot be compensated by adjusting the port injection pulse width, then the controller may selectively reactivate the direct injector coupled to the cylinder and enable the remaining fuel mass to be made up for via direct injection on the same engine cycle. For example, the controller may maintain the original port injection and provide the entirety of the fuel error via direct injection. Alternatively, a portion of the fuel error may be compensated via adjustments to the port injection pulse width, while a remainder of the fuel error is compensated via direct injection on the same engine cycle. Further still, if the additional fuel mass to be compensated via direct injection is lower than the minimum pulse width of the direct injector, the direct injector may be maintained disabled and the additional fuel mass may be compensated via port injection on the subsequent engine cycle, such as by increasing the pulse width of the port injector on the subsequent engine cycle.
In this way, lean combustion events triggered by a tip-in request received late within a port injection cycle can be reduced. The technical effect of enabling direct injection to be selectively re-enabled in response to a tip-in when originally operating with port injection only is that a late decision to increase fuel mass to a cylinder can be accommodated without degrading engine performance. In addition, by compensating a port injection fuel error via direct injection on the same engine cycle, the need for open valve injection from a port injector is reduced. In addition, the use of direct injection, while occurs during the intake or compression stroke, is that air-fuel mixture formation is improved as compared to when the fuel is delivered via open intake valve port injection.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.